Microelectromechanical systems (MEMS) based pressure sensors are being incorporated into such diverse equipment as medical instruments, laboratory instruments, industrial equipment, and automotive circuitry. A significant number of applications are becoming important where pressure sensor devices are exposed to harsh environments. Such environments include solvent mixtures (e.g., fuel), water (e.g., salt water), acids, and bases. End users of pressure sensors have found that standard devices fail in such environments.
For example, one type of pressure sensor measures absolute pressure. The absolute pressure sensor is made by forming a cavity within a first silicon wafer and then attaching a second silicon wafer to the first wafer and thinning the second wafer above the cavity, thereby providing a diaphragm over a sealed chamber. Piezoresistors formed on or adjacent to the diaphragm measure pressure by sensing how much the pressure acting on the front side of the diaphragm deflects the diaphragm into the sealed chamber. In this example, upon exposure to a harsh environment, corrosion often occurs, for example, at interconnects of the piezoresistors, which causes the absolute pressure sensor to degrade or fail.
Accordingly, a continual need exists in the art for pressure sensors that are capable of being used in harsh environments.